1. Field of Invention
The present invention relates generally to spectrometers and interferometers and, more particularly, to lamellar gratings used in spectrometers and/or interferometers.
2. Discussion of Related Art
Spectrometers require components to generate light, disperse the light, and detect the light. To disperse the light there are two common approaches; (1) using a fixed grating to separate the wavelengths by diffraction and (2) using an interferometer to select wavelengths using an interferometric filter. Each approach has its own advantages and drawbacks. The grating dispersion spectrometer requires an input slit, which limits the resolution or throughput of the spectrometer. Narrower slits give higher resolution and lower throughput, while wider slits give lower resolution and higher throughput. It is well known that for a given size of optics, an interferometer-based spectrometer can have better signal-to-noise than a scanning-grating spectrometer, due to the throughput (or Jacquinot) and multiplex (or Fellget) advantages offered by interferometer-based spectrometers. The throughput advantage comes from a larger input optic (because there is no slit) and the multiplex advantage comes from measuring many wavelengths at one time.
The interferometer-based spectrometer characteristics depend on the interferometer dimensions and thus its free spectral range. For an interferometric filter operating in its fundamental mode with maximum free spectral range, the throughput is large because there is no slit to limit the amount of light entering the instrument. However, since only one wavelength is passed through the interferometric filter at any one time, it does not have a multiplexing advantage. Conventional Fourier transform spectrometers with large optical cavities pass many wavelengths at any one time and thus offer throughput advantages from not having a slit as well as a multiplexing advantage from passing many wavelength of light at any one time. In addition, Fourier transform spectrometers also have the advantage of a built-in internal wavelength reference, typically a He—Ne laser. These advantages have made the Fourier transform Spectrometer the laboratory instrument of choice in the infrared region of the spectrum. However, interferometers with large optical paths generally require large rigid structure to maintain the stability necessary to function properly. This makes the instruments large, heavy, and sensitive to environmental factors such as temperature fluctuations and mechanical vibrations. They are not well suited to building hand-held spectrometers.
These limitations arise primarily from the fact that Fourier transform spectrometers use Michelson interferometers in which the optical paths are at right angles, as shown in FIG. 1. Referring to FIG. 1, in a Michelson interferometer, incident light from an input 102 is split by a semi-transparent mirror 114 into two paths 104 (Path 1) and 106 (Path 2), thus providing two paths from the light source at the input 102 to the detector at the output 108. Light in the first path 104 reflects off the semi-transparent mirror 114, goes to the top mirror 110, is reflected back, and passes through the semi-transparent mirror, to the output 108. In the second path 106, the light passes through the semi-transparent mirror 114 to the mirror 112 on the right, reflects back to the semi-transparent mirror, then reflects from the semi-transparent mirror into the output 108. These two separate paths 104, 106 for the light beams that interfere make for critical alignment and sensitivity to environmental factors.
Another type of interferometer including a lamellar grating is shown in FIG. 2. The lamellar grating interferometer comprises a reflective lamellar grating system 202 in which half the mirror elements 202a are movable, as illustrated in FIG. 2. The mirror elements also include a reflective surface 204. In this type of interferometer the two optical paths 104, 106 for the interfering beams are almost identical. This greatly reduces the criticality of the alignment and the sensitivity to environmental factors. Such interferometers have been used in Fourier Transform spectrometers for the far-infrared and Terahertz region of the spectrum, as disclosed in a paper by J. Strong and G. A. Vanasse entitled “Lamellar grating far-infrared interferometer,” published at J. Opt. Soc. Am. 50, 113 (1960). For such long wavelengths, the lamellar grating structure can be fabricated with conventional machining techniques. However, as the wavelength gets shorter (the mid and near infrared) such conventional fabrication techniques can no longer make the small components and achieve the necessary tolerances.
Recently microelectromechanical processes (MEMs) or micro-fabrication, which use silicon micromachining techniques, have been used to build a visible/near infrared spectrometer based on the lamellar interferometer configuration of FIG. 2, as disclosed in a paper by O. Manzardo, R. Michaely, F. Schddelin, W. Noell, T. Overstolz, N. De Rooij, and H. P. Herzig, entitled “Miniature lamellar grating interferometer based on silicon technology,” published at Opt. Lett, 29, 1437 (2004). This paper is referred to herein as Reference 2, and is herein incorporated by reference. Similarly, a lamellar grating interferometer used as a Fourier transform spectrometer based on a diffraction grating operating in the zeroth order is disclosed in U.S. Patent Pub. No. 2007-0159635 filed Jan. 12, 2006 and entitled “Fourier transform spectrometer.” Such a spectrometer is now available commercially from ArcOptix (www.arcoptix.com). Such micro-fabrication techniques are capable of achieving the desired precision and small dimensions for lamellar grating interferometers in the visible and near infrared part of the spectrum, but are limited in the size of the grating that can be fabricated by these techniques.
The throughput (or Jacquinot) advantage of Michelson interferometers comes from the much larger optical beam diameter that can be achieved compared to a dispersive instrument, which is limited by the area of its slit. For example, a typical dispersive instrument may have a slit that is about 5 millimeters (mm) high and about 0.1 mm wide for a total area of 0.5 mm2. By contrast, a Michelson interferometer of comparable size and resolution may have a beam diameter of about 20 mm, or an area of 314 mm2. This large difference in area is a primary reason why Fourier transform interferometers (using Michelson interferometers) have become the laboratory standard for the mid-infrared region where light sources and detectors perform relatively poorly.
Lamellar grating interferometers also possess the throughput advantage. However, the micro-fabrication technology described above is limited in its ability to produce lamellar gratings with large cross sectional areas. The instrument described in Reference 2 has a lamellar grating that is 0.075 mm high by 3.2 mm long, thus having a total cross sectional area of 0.24 mm2. The planar silicon surface micromachining used to produce this grating makes it is difficult to build structures perpendicular to the planar surface that are significantly higher than the 0.075 mm reported in Reference 2.